Typically, circuit boards that require protection from moisture, electric leakage and dust are coated with moisture proof insulative films, known as conformal coatings, such as, acrylic, polyurethane, silicone or epoxy synthetic resins dissolved in a volatile solvent. When applied to clean circuit boards, an insulative resin film of uniform thickness and without pinholes, is formed as the solvent evaporates.
Spraying is the most commonly used insulative coating method employed in mass production. Spraying can be categorized as either air spraying in which an air stream is impinged against the stream of liquid coating material after leaving the spray nozzle to form an atomized spray pattern, or airless spraying in which the coating material is dispensed as a nonatomized spray pattern, as is disclosed in U.S. Pat. No. 5,294,459, assigned to Nordson Corp., which is hereby incorporated by reference in its entirety.
In airless spraying of conformal coating material, as described in the U.S. Pat. No. 5,294,459, the coating material is sprayed onto a circuit board as a flat, nonatomized pattern. Relative movement is effected between the nozzle and the circuit board in a direction transverse to the plane of the flat pattern discharged from the nozzle. The supply of coating material to the nozzle is intermittently interrupted so as to prevent a deposit of liquid coating on regions of the circuit board and/or circuit components which are to be left uncoated.
While air spraying, in which an air stream is impinged against the stream of liquid coating material to atomize the material after leaving the spray nozzle, as disclosed in the U.S. Pat. No. 5,294,459, has proven to be an effective means of applying the material to a substrate, there is still some overspray which increases the production costs of the circuit boards. Even with airless spraying, the distance of the nozzle to the circuit board sometimes causes the coating material to coat regions which are to be left uncoated.